Diana enjoyed a close relationship with her sons before her tragic death in 1997. The Princess wanted her children to have as normal an upbringing as possible and treated them to days out at theme parks, sent them to public school and took them on royal tours.
In turn, William and Harry have honoured their mother's legacy by continuing some of her most vital charity work.
HELLO! takes a look back at the Princess' sweetest moments with her children. From personal photos and public appearances, it was clear that Diana adored her sons. They were the apple of her eye, and likewise, William and Harry have spoken lovingly about their mother.
In an ITV documentary Diana, Our Mother: Her Life and Legacy, which aired in 2017, Harry said: "She was our mum. She still is our mum. And of course, as a son I would say this – she was the best mum in the world. She smothered us with love, that's for sure."
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WATCH: Princess Diana's cutest mum moments with her sons William and Harry
Diana was the best at giving hugs. Harry has said: "She would just engulf you and squeeze you as tight as possible and being as short as I was then, there was no escape, you were there for as long as she wanted to hold you. Even talking about it now, I can feel the hugs that she used to give us.
"I miss that feeling, I miss having that mother to give you those hugs and that compassion that everyone needs. Behind closed doors, she was a very loving, caring mother and an incredibly funny person.
"I think she lived a lot of her life, especially in private, through us and I think that childish fun element really came out when she was spending time with us."
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Diana did her best to keep a young William and Harry out of the public eye, and tried to keep their undeniably unusual upbringing as normal as possible – whether that meant doing the school run or taking them to Alton Towers for a treat. In the ITV documentary, Harry revealed that Diana was "one of the naughtiest parents".
"She was fun through and through," he said. "One of her mottos to me was, 'You can be as naughty as you want, just don't get caught.' She would come and watch us play football and, you know, smuggle sweets into our socks."
Throughout their lives, Princess Diana kept the two boys grounded.
From teaching them how to play piano at Kensington Palace to taking them to their first days at school in adorable matching red school uniforms, the philanthropist had countless duties but always had time for her children.
After welcoming her second son, Prince Harry, in 1984, Diana revealed that her two boys adored each other in a letter to a palace steward, Cyril Dickman.
She wrote that William "could not stop kissing" Harry after his birth, adding: "William adores his little brother and spends the entire time swamping Harry with an endless supply of hugs and kisses, hardly letting the parents near!"
She also updated Cyril with their boarding school education a few years later, writing: "The boys are well and enjoying boarding school a lot, although Harry is constantly in trouble!"
We absolutely love this photo of Diana and a young Harry. "She smothered us with love, that's for sure," the Prince has said.
Playtime at home was always fun, and dressing up was very much encouraged!
William looked absolutely adorable on his pony, while his younger brother Harry pulled the reins.
Diana also enjoyed taking her children on holidays. Here, the family are pictured on Necker Island.
While the Princess was, and still is, known as the ultimate style icon, the same may not be said for her sons William and Harry when they were growing up.
"I genuinely think she got satisfaction out of dressing myself and William up in the most bizarre outfits – normally matching. It was weird shorts and little shiny shoes with the old clip on.
"Looking back at the photos it just makes me laugh and I think, 'How could you do that to us?'" Harry laughed.
William has revealed he often speaks about 'Granny Diana' to his children George, Charlotte and Louis.
Of what kind of grandmothert she would have been, he says: "She'd be a nightmare grandma, absolute nightmare. She'd love the children to bits, but she'd be an absolute nightmare.
"She'd come and go, she'd probably come at bath time, cause an amazing amount of sea and bubbles and bathwater all over the place and then leave."
The boys often went on ski holidays with their mum - some memories that they'll cherish forever.
The children also went on royal tours from a young age, often matching in the same adorable outfits.
Diana was adamant that her children would have a normal childhood. She insisted that her children be educated outside the palace walls, so when the time came for William to start nursery, she enrolled him at Mrs. Mynors Nursery School in west London.